(physics) An electrically charged particle, either an electron or an ion, emitted by a conducting material at high temperatures

Concerning the emission of electrons from a heated electrode.

plural of thermion

In a thermionic manner.

The science dealing with thermionic emission.

(chiefly historical) An electronic device incorporating electrons from a glowing cathode inside an evacuated glass tube, functioning as a switch, amplifier etc.

plural of thermionic valve